1. Field of the Invention
The present invention relates to notch filters, and particularly to a low frequency notch filter integrated circuit having digitally controlled resistor ladders.
2. Description of the Related Art
Very low frequency filters have a wide range of applications in biomedical signal processing, and in audio and neural networks. In instrumentation systems, it is usually desired to eliminate power line frequency disturbance of 60 Hz (50 Hz in Europe) from the measured signal using notch filters. Examples include biomedical engineering, automatic control, and communications over public utility systems. In particular, power line interference is the most common problem in the detection and processing of biopotential signals. Despite the use of differential amplification methods and active body potential driving to eliminate the common-mode signals, line frequency interference occurs in the important frequency range where biopotentials and other physiological signals have most of their energy. This is the case in electroencephalogram (EEG), electrocardiogram (ECG), and electromyelogram (EMG) recordings.
Power line interference has considerable effect and plays an important part on the quality of these signals. Many notch filter circuits are designed to eliminate this unwanted noise. In order to utilize very large-scale integration (VLSI) techniques in biomedical instrumentation, implementation of a 60 Hz notch filter in a single integrated chip (IC) is desired. This has been a challenging design problem due to the difficulty in developing efficient methods to achieve a large time constant using integrated passive elements. Such a notch filter would require resistances in the order of megaohms and capacitances in the range of nanofarads, and therefore is impractical for implementation on an integrated circuit (IC) chip. The available silicon area on an IC chip limits the sizes of resistors and capacitors. Typical values for on-chip integrated circuit resistors are from 1Ω to 40 kΩ, and for capacitors, from 0.5 pF to 50 pF. Components with much larger values would suffer from nonlinearities and parasitics that may prevent proper circuit operation.
Thus, a low frequency notch filter integrated circuit solving the aforementioned problems is desired.